get_sidebar should be able to accept an array of names as parameter.

Description

get_sidebar uses load_template to do the work of template loading.
load_template will accept an array of template names for loading.
load_template will then load the first existing template from that array.

Seems like get_sidebar should also accept an array of names that can be searched in declining order.

This would allow a theme to specify a first choice, second choice, etc. for the sidebar.